~ RG1 ~ RG2 ~ RG3 COORDINATION OF REGULATION SYSTEMS FOR GENERATORS AND TRANSFORMERS IN AN INDUSTRIAL COMBINED HEAT AND POWER PLANT 1.

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1 49 COORDINATION OF REGULATION SYSTEMS FOR GENERATORS AND TRANSFORMERS IN AN INDUSTRIAL COMBINED HEAT AND POWER PLANT 1. INTRODUCTION All the national combined heat and power plants work with the national power system; usually they are connected to a 110 kv network. Generator transformer units in combined heat and power plants are connected to 110 kv busbars, because the power for their own needs is small compared with the power transferred to the system. Power received for technological purposes from industrial heat and power plants is comparable to the generated power. Generators (usually there are several of them) in industrial heat and power plants work in parallel, connected to 6 kv busbars. 6 kv busbars are connected to the 110 kv station with at least two (usually several) 110/6 kv transformers. Sometimes these are three-coil transformers aimed at reducing fault currents on the side of 6 kv. An example diagram of an industrial heat and power plant is shown in fig kv 6 kv RT1 RT2 RT3 ~ RG1 ~ RG2 ~ RG3 Fig. 1. Example diagram of industrial heat and power plant In accordance with the requirements of network operators, generators and transformers are equipped with control systems. The experience gained by the author in several industrial plants brings him to the conclusion that the generator control systems are often incorrectly adjusted, and the algorithms of transformer 110/6 kv controllers are wrong. The article presents a proposal for coordination of algorithms used in control systems. 2. GENERATOR CONTROL SYSTEMS Generally, all generators are equipped with systems for voltage regulation with current and power angle limiters. All control systems have a current compensation system, which is sometimes very simplified (depending on the controller supplier). Tab. 1 shows example data from the controllers in one of the industrial heat and power plants. Abstract Generators (usually there are several of them) in industrial heat and power plants work in parallel, connected to 6 kv busbars. 6 kv busbars are connected to the 110 kv station with at least two (usually several) 110/6 kv transformers. Generator control systems are often incorrectly adjusted, and the algorithms of 110/6 kv transformer controllers are wrong, making cooperation between the control systems in the plant impossible.the article presents a proposal for coordination of algorithms used in control systems.

2 50 Tab. 1. Generator control systems No. Symbol S n [MVA] P n [MW] Type LPS LPW LKM PSS Z k X k R k 1 TG Digital YES YES YES YES Rk+jXk YES YES 2 TG Digital NO YES YES NO YES NO 3 TG Digital NO YES YES NO YES NO S n, P n rated apparent and active power RN generator controller LPS, LPW, LKM limiter of stator and rotor current and power angle PSS system stabilizer Z k current compensation impedance Z k = R k + jx k A current compensation system makes the value of obtained voltage conditional on the load value (mainly reactive power) and allows a stable distribution of reactive power between cooperating generators, even in the case of cooperation of directly connected (without reactors) sections of busbars. As shown in tab. 1, TG1 has a full current compensation system, whereas TG2 and TG4 are only dependent on reactive power (part X k ). TG1 has all limiters. TG2 and TG4 have no stator current limiters. The present settings of power angle limiters (reactive power consumption): Q 0 of all generators are incorrect because they do not use the possibility of reactive power consumption by generators. Current compensation settings are mutually uncoordinated, which is caused by the fact that generators are started by various teams at several-year intervals. A similar condition was found in many industrial heat and power plants. Current use of control systems When adjusting the reference voltage of generators, the control room staff can control the proper distribution of reactive power, approximately proportional to the rated apparent power. With the adjusting cooperation of control room staff, the control systems are responsible for the following tasks: maintaining the traditionally set reference voltage on 6 kv busbars providing an approximate correct distribution of reactive power between cooperating generators. For the proper performance of the second task without constant intervention by the control room staff, it is necessary to set an equal in relative units current compensation of controllers in all generators. Correct properties Q-U of generator control systems, ensuring a proportionate or reference distribution of reactive power between generators In the task of controlling voltage, the modern generator controllers have the following properties U = f(q): Without current compensation with a very slight gradient resulting from finite value of amplification in the open loop of voltage control circuit. With current compensation introducing a virtual replacement impedance between 1 generator and controller. If generators do not run connected in parallel to the same busbar system, or their busbars are always separated by a reactor, they can operate with current compensation set to 0 (zero). If generators run connected in parallel to the same busbar system, or their busbars are not always separated by a reactor, two solutions may be used: 1. Assuming that the accuracy of digital controllers is very high and the aforementioned gradient without current compensation is sufficient, the current compensation should be set to 0 (zero). Also, the distribution of reactive power between generators without participation of control room staff should be observed. If the distribution of reactive power is approximately proportional to the apparent power 1 Some controller suppliers offer only simplified current compensation, which provides only virtual reactance.

3 51 in generators during changes in load, the first solution should be used. Otherwise the second solution solution should be selected. 2. Current compensation should be set to e.g. X k = 0.02 (2%), then the distribution of reactive power between generators without participation of control room staff should be observed. If the distribution of reactive power is approximately proportional to the apparent power in generators during changes in load, the above solution should be used. Despite the high accuracy of digital controllers, the unsystematic deviations in voltage transformers may require the use of the above solution. Due to the lack of or inaccurate information on the value of real strengthening in the open loop of a control system, an experimental procedure according to point 1 or 2 is necessary. In the unlikely event that at the value of X k = 0.02 pu the distribution of reactive power is not approximately proportional to the apparent power in generators, the value X k should be increased to X k = 0.03 pu. The method for setting generator controllers described above should ensure that the voltage in busbars is maintained with high accuracy and without interference of transformer operators and controllers. 3. TRANSFORMER CONTROL SYSTEMS Current state Usually, all transformers are equipped with voltage controllers that operate tap switches. The used controllers come from different manufacturers, but in principle, according to the documentation of one of the producers, they are designed to automatically control the voltage of lower side or the number of transformer (transmission) tap. Such controllers are designed to be used in 110/15 kv transformers located in stations supplying the 15 kv distribution network, the so-called switching stations. Controllers have no components that ensure mutual coordination during parallel operation of transformers. Algorithms for operation of such controllers prevent cooperation with the generator control systems on the side of 6 kv. In industrial heat and power plants such transformer controllers are always off. The off state is correct because probably because of poor project documentation transformer voltage controllers with incorrect operation algorithms have been used. The algorithms used in controllers are for voltage regulation appropriate for e.g. 110/15 kv transformers powering medium-voltage distribution networks. The designer probably designed a transformer with controller without considering the specificity of three transformers connecting the 6 kv network in heat and power plant with the 110 kv network. Presently the tap switches of transformers in industrial heat and power plants are controlled manually by control room staff, which controls the appropriate exchange of reactive power with the 110 kv network and proper distribution of reactive power between cooperating transformers. Correct properties Q-U of 110/6 kv transformer control systems, ensuring a proportionate or reference distribution of reactive power between transformers and proper cooperation with generator control systems Assumptions Transformer regulators should control the tap switches in all 110/6 kv transformers without constant interference by the control room staff. The operation algorithm should ensure: Proper cooperation between transformers and reactive power distribution in approximation proportional to the rated power in particular transformer coils Maintaining the exchange of reactive power with the 110 kv network of an average value minimizing the tariff charges for readings of kvarh meters That the acceptable tap switching frequency is not exceeded. Under normal conditions, the task of maintaining the set voltage on 6 kv busbars is performed by generators control systems.

4 52 Characteristics of transformer controllers 110/6 kv transformer controllers should have characteristics that ensure: Maintaining the reference value of reactive power in range Q trn,q trm if the voltage on the 6 kv side is within U trn,u trm Taking over the task of maintaining voltage on the 6 kv side after exhausting the generator capability The assumed distribution of reactive power, proportional to rated power of respective transformer windings. Controllers offered by the better suppliers have components ensuring such cooperation. The characteristics proposed for transformer controllers are shown in fig. 2. Dead bands and time delays Dead bands, independent of variables U and Q, should ensure stable operation of the control system at the borders of the O area in fig.2. Time delays For variable Q, they should ensure that the allowed frequency of tap switching is not not exceeded For variable U, they should ensure that tap switches are controlled as quickly as possible. The need for action in order to maintain a safe voltage for the plant is rare and fast switching will not cause a significant increase in the number of switches. The priority ensuring a proper quality of electricity that determines the continuity of plant operation is obvious. U tr - - U trzm U trzm Q mtr Q Mtr Q tr Fig. 2. Characteristics of transformer with controller Q tr transformer reactive power received from the 110 kv network U tr transformer voltage Ut rzm, Ut rzm lower and upper reference value of transformer voltage Q Mtr reduction of reactive power consumption from the network Q mtr reduction of reactive power supply to the network the area where transformer controllers are not running increasing voltage on the 6 kv side reducing voltage on the 6 kv side absence of transmission switching. 4. GUIDELINES FOR CHANGES IN SETTINGS AND IN ALGORITHMS FOR GENERATOR CONTROL SYSTEMS IN HEAT AND POWER PLANTS AND 110/6 KV TRANSFORMERS Assumptions Synchronous generator and 110/6 kv transformer control systems in heat and power plants should cooperate and ensure: Maintaining voltages that guarantee the proper operation of all receivers connected with the technological process in the plant

5 53 Reducing consumption of reactive power from the 110 kv network for the purpose of minimizing the costs of purchasing electricity from the 110 kv network The possibility of using the entire area of acceptable generator states, taking into account all limitations (stator current, rotor current, power angle, temperature of extreme stator packages) Proper (approximately proportional) distribution of reactive power between cooperating generators Proper (approximately proportional) distribution of reactive power between cooperating transformers That the acceptable transformer switching frequency is not exceeded. Characteristics of generators The following are important elements in modern generator controllers: Z k = R k + jx k I g = I P + ji Q z0 Fig. 3. Equivalent diagram of generator with controller with Zk Current compensation system providing equivalent diagram in the area of steady states is shown in fig.3. z0 reference voltage in generator at idle run generator voltage Z k = R k + jx k current compensation impedance. Current compensation impedance Z k = R k + jx, with adjusted value in four quadrants of complex plane allows the introduction of virtual voltage measurement and almost any shape of equivalent circuit (steady state) in the controller in the diagram of a power subsystem of heat and power plant. Stator current limiter ensuring that the acceptable value of stator current is not exceeded Rotor current limiter ensuring that the acceptable value of rotor current is not exceeded Power angle (reactive power consumption) limiter that ensures stable cooperation with the system and, sometimes, prevents the extreme parts of stator core from heating. z0 Q g Fig. 4. Characteristics of generator with controller Q g generator reactive power generator voltage z0 reference voltage in generator at idle run stator or rotator current limitation power angle limitation Generator characteristics on the plane Q, U with control system containing the aforementioned elements show the relation U = f(q) with limitations controlled by limiters, fig. 4.

6 54 Combination of generator and transformer characteristics Combination of the characteristics of generator and transformer control systems, which implement the algorithms described above, gives the resultant characteristics, shown in fig. 5 z0 U trzm U trzm 0 Q g, Q tr, Q Σ Range Q g Fig. 5. Combined characteristics of generators and transformers Q g, Q tr,, Q Σ total reactive power of generators and transformers voltage in 6 kv busbars z0 reference voltage at idle run stator or rotator current limitation power angle limitation U trzm reference value of the highest voltage in transformers on the 6 kv side U trzm reference value of the lowest voltage in transformers on the 6 kv side resultant characteristics of generators and transformers fuzzy part of the resultant characteristics the area where transformer controllers are not running 5. DESCRIPTION OF COOPERATION BETWEEN CONTROL SYSTEMS OF GENERATORS AND TRANSFORMERS Normal state In normal states, the generator and transformer controllers run autonomously. Generator regulators with the reference voltage occasionally adjusted by the control room staff ensure the correct distribution of reactive power between cooperating generators. Generator controllers maintain the reference voltage in 6 kv busbars (fig. 4 and 5) in the full range of reactive power generation or consumption from limitation of power angles to current reduction (within the acceptable states). Transformers controllers maintain reactive power consumption (fig. 2 and 5) within the limits ensuring avoidance of charges for readings of kvarh meters (the so-called reactive power). Abnormal state After reaching the boundaries of acceptable states, the generator controllers are not able to maintain the reference voltage on 6 kv busbars (fig. 3 and 5) as a result of working power angle limiters or current limiters. After exhausting the ability to maintain reference voltage by generators, the task of maintaining reference voltage is assumed by transformer controllers (fig. 2 and 5). Emergency During emergencies such as short circuits in 110 kv and 6 kv networks, short-term voltage dips, etc., the transformer control systems should not be running. Generator control systems must react quickly, according to the current characteristics, in order to maintain the stability of cooperation between the heat and power plant and the power system, as well as to ensure proper operation of power protection systems.

7 55 6. PROPOSED CONCEPT OF PRIMARY SYSTEM COORDINATING THE COOPERATION OF ALL GENERATOR CONTROLLERS IN HEAT AND POWER PLANTS AND 110/6 KV TRANSFORMERS Local demand for reactive power is met from the following sources: Generators of heat and power plants The 110 kv network (by 110/15 kv transformers) Capacitor banks in the 6 kv network. The selection of characteristics and settings in control systems of generators and 110/6 kv transformers described above provides the correct voltage on 6 kv busbars and correct value of reactive power received from the 110 kv network. Capacitor banks are controlled manually from the control room. It is possible to develop a control algorithm for the primary system which includes all the mentioned sources; however, this algorithm should be based on technical and economic analysis: Optimal voltage on 6 kv busbars, taking into account the voltage drops 6 kv busbars receivers relation Optimal use of capacitor banks Optimal use of exchange of reactive power with the 110 kv network, not only for the purpose of avoiding charges A proper selection of settings in 6/0.4 kv transmissions in transformers. The algorithm mentioned above can be included in the manual for the control room (to be implemented by the staff on duty), or implanted in the primary control circuit. Such systems are used in the country in production nodes, the so-called GCPN (Group Control of Production Node). Domestic suppliers offer such systems, which can execute algorithms required by the ordering parties. 7. SUMMARY AND CONCLUSIONS Generator controllers In general, the present generator controllers described above do not require replacement. Using appropriate settings, these controllers can implement the algorithms described above and properly cooperate with new transformer regulators. Transformer controllers The currently used controllers are designed to automatically control the voltage of lower side or the number of transformer tap. Typical controllers have no components that ensure mutual coordination during parallel operation of transformers. Algorithms for operation of these controllers prevent cooperation with the generator control systems. These controllers should be replaced with the controllers executing the algorithm described in the article. Transformer controllers are currently offered on the domestic market, and are used only for the purpose of voltage regulation. No controller executes the proposed algorithm. Domestic suppliers should offer controllers that execute the proposed algorithm for control of reactive power in the specified range of 6 kv voltage value and control of voltage at the borders of this range. Primary control system The decision to use a primary GCPN (Group Control of Production Node) system in a particular heat and power plant should be made after the preparation of technical and economic analysis that takes into account the benefits of optimizing the voltage levels and reactive power management in the entire facility, including the plant. Domestic suppliers offer such systems, which can execute algorithms required by the ordering parties. BIBLIOGRAPHY 1. Work of the Department of Electrical Power Engineering at Gdańsk University of Technology carried out within the research project Power System Security, Hellmann W., Szczerba Z., Regulacja Częstotliwości i Napięcia w Systemie Elektroenergetycznym, WNT Machowski J. et al., Power System Dynamics Stability and Control. J. Wiley, Szczerba Z. et al., Poradnik Inżyniera Elektryka, rozdział 7, Systemy Elektroenergetyczne, WNT 2005.